Writing Simple Chemical Formulae 2 - using prefixes

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The names of some compounds have prefixes in them e.g. carbon dioxide has a prefix 'di-'. This prefix is used to indicate the number of atoms of the element in the compound.

Prefix Meaning Example
Mon- or Mono- 1 Carbon monoxide
Di- 2 Carbon disulphide
Tri- 3 Sulfur trioxide
Tetr- or Tetra- 4 Carbon tetrachloride
Pent- or Penta- 5 Phosphorus pentoxide
Hex- or Hexa- 6 Sulfur hexachloride
Hept- or Hepta- 7 Iodine heptafluoride

No valency number are needed to write these formulae. In some ways, they are the easiest to write.

Carbon monoxide contain carbon and one atom of oxygen, so formula is CO

Carbon disulfide contains carbon and two atoms of sulfur, so formula is CS2

Sulfur trioxide contains sulphur and three atoms of oxygen, so formula is SO3

Iodine heptafluoride will have the formula IF7

Two prefixes in one formula

Dinitrogen tetroxide contain two atoms of nitrogen and four atoms of oxygen and will have the formula N2O4

(Do not cross these numbers over, and don't simplify them as this would give NO2 which is a different compound called nitrogen dioxide)

If you're wondering why there are sometimes two versions of the prefix in the table above, the choice simply depends on the element the prefix goes in front of. If it starts with a consonant, use mono, tetra, penta, hexa and hepta e.g. lead tetrabromide. If it starts with a vowel, use mon, tetr, pent, hex and hept e.g. lead dioxide.

P.S.  You thought this was a chemistry tutorial not one to help 'English'